NEW TECHNOLOGY: Adding big battery turns non-spin peaking GT into spinning reserve

On the one hand, if this Southern California Edison (SCE) facility is a harbinger, the industry could be entering a golden era of creative, innovative grid-scale storage applications. On the other hand, California is such an anomaly, this golden era, at least from a national or global perspective, could still be years away.

Either way, SCE gets credit for pioneering the world’s first commercial applications of a hybrid 10-MW/4.3-MWh lithium-ion battery energy storage system (BESS) and LM6000 nominal 50-MW peaking gas turbine/generator, dubbed the hybrid enhanced gas turbine (EGT) by system supplier, General Electric Co, Schenectady, NY (Fig 1).

Lest you think this is a BESS simply sited at a peaking GT facility, it’s actually a fully integrated BESS/GT system, thanks to a control system having many proprietary details.

Wellhead Power Solutions LLC, Sacramento, rounds out the partnership for the project (Sidebar).

In a way, it’s back to the future for SCE in pioneering grid-scale storage. The utility, with EPRI support, demonstrated what was at the time the utility industry’s largest BESS (lead-acid battery chemistry) at its Chino substation from 1988 to 1996. With this latest system, at its Center Peaker facility (Fig 2), and a companion Hybrid EGT at another of its five peaking facilities, SCE is again at the forefront of grid-scale storage technology.

From non-spin to spin

The core value proposition for the hybrid EGT at Center, Norwalk, Calif, is capitalizing on the pricing for spinning reserves in the CAISO market, according to SCE’s Vibhu Kaushik, which can be from 50 to 70 times higher than the non-spinning reserve price, while also significantly reducing emissions and optimizing the operation of other system assets. The LM6000 has been there since 2007 functioning as a peaker.

Essentially, the battery’s main job is to dispatch power immediately upon receiving the CAISO dispatch signal to the plant, and keep delivering power for the first few minutes as the GT/G starts up. Typically, the GT begins its startup sequence once the battery achieves 250 kW output. Once the GT/G is delivering significant power to the grid, it also begins to recharge the battery.

CAISO can modulate the unit’s output from between 0 and 49 MW at a moment’s notice. The battery is able to take load swings while the GT/G remains at max output, it’s most efficient and lowest emissions operating point. Another aspect of the project was to reduce the LM6000 start duration from 10 minutes to five minutes. Thus, the BESS delivers megawatts immediately and ramps extremely fast, while the gas turbine takes the first two minutes to synchronize, and then less than three minutes thereafter to achieve full output of 49 MW.

In the absence of the BESS, the GT/G could be “spinning,” ready to put megawatts onto the grid when called, but this practice consumes fuel and discharges emissions while not being productive. Other units around the system also function non-optimally; combined cycles, for example, could be producing at 90% of capacity, holding 10% back as spinning reserve. Having BESS units take up this slack allows combined cycles to continue to operate at their optimum design/maximum output point.

SCE Project Manager Matthew Zents notes, “essentially, the hybrid EGT gives us a more efficient asset that can participate in a market ‘sweet spot’ for dispatch below 49 MW.”

The other big factor in justifying the project was the catastrophic leakage from the Southern California Gas Co’s (SoCalGas) Aliso Canyon gas storage field and subsequent shutdown of the facility. When that occurred, the LA Basin, and southern California in general, was at risk of natural-gas shortages. In fact, SCE was under a threat of gas curtailment most of last winter.

Having a BESS at a peaker was seen as a means of partially alleviating that risk. Since a fully charged BESS didn’t require a gas fuel feed, unlike a peaking GT/G, it could at least supply critical loads for short periods of time.

Although spinning reserves and mitigating the Aliso Canyon risk were mainly how the project was justified, Kaushik and Zents concede that neither CAISO nor SCE can fully predict how the hybrid EGT will be dispatched. “This is new territory,” said Zents, “we’re all going to learn how such a unit best contributes to grid management.”

In grid-scale storage parlance, the Center BESS is what’s known as a “power” application. The battery’s output can be drained in 20 minutes. In “energy” applications, the battery’s output is available over several hours or more.

As for the one challenge which plagues even the strongest advocates of Li-ion BESS for grid-scale storage, the risk of catastrophic fire, SCE indicates Center is equipped with Fike Corp’s FM200 clean-agent fire suppression system, which is now considered the standard for a large-scale BESS. According to Fike literature online, it uses HFC 227ea suppression agent and is able to discharge in 10 seconds or less.

Wellhead’s role in integrated GT/battery development and implementation

Wellhead Electric Co is a small, 30+-year-old IPP developer, owner, operator, and constructor, based in Sacramento (65 employees, 400 MW. Affiliate Wellhead Power Solutions LLC (WPS) performed the installation of the Hybrid EGT and designed and managed the related upgrade of the SCR at SCE’s Center Peaker facility. WPS has been granted the patent for the hybrid technology embedded in the SCE Center project (U.S. Patent No. 9,722,426 for the Hybrid Energy System and Method). 

The patent covers the company’s innovative hybrid system integrating gas turbines with battery power systems that enable electrical generating stations to provide advanced grid stability. The specific GE LM6000 Hybrid Electric Gas Turbine (EGT) controls were co-developed by WPS and GE based on Wellhead’s hybrid patent.

GE’s LM6000 Hybrid EGT is governed by a Hybrid Control System (HCS) which produces a blended high-performance output while managing the Battery State of Charge (SOC) and GT starting/stopping, and providing precise megawatt (net) control. 

Wellhead has a commitment to powerplant and electric industry technology development with four additional technologies in the pipeline.  It began developing and experimenting with EGT concepts on its LM6000 units in 2010.  The development effort was prompted by a dramatic increase in variable operation of its peaker fleet for grid reliability reasons (versus peak energy) as renewables generation increased in California. 

For the hybrid functionality to be fully effective, the system must ensure that the unit provides the same droop response with or without fuel burn, and complex programming is required to allow for the necessary bidirectional sequencing, adaptive synchronization, precision megawatt control, advanced emissions controls, and modified fuel shut-off/purge system of the gas turbine.  This integrated control approach allows for superior grid support with a Pmin of 0.0 MW and no minimum up time or down time requirement on the gas turbine.

Switchgear, controls challenge initial ops

Zents and Operator/Mechanic Rick Snyder reported that the unit has met all of its performance obligations and technical objectives since commissioning in March. The plant staff is still tweaking the control system, the source of the project’s most important lessons learned. The facility can be remotely controlled from SCE’s Eastern Operations Generation Control Center in Redlands, but also controlled locally should the need arise.

Center proved what many other grid-scale storage projects have experienced; controls, automation, and digital communication are the greatest challenges with grid-scale BESS. “We learned that getting the main controller to ‘talk’ to the Mark VI GT control system, the Woodward system, and other sub-systems was anything but straightforward,” observed Zents.

In addition, there are numerous communication interfaces: The control system handles battery operation and life management, interfaces with the grid, modulates between the BESS output and the GT output, and optimizes among everything on a sub-second basis. Fine-tuning digital communications among all the components and functions took time and patience.

As just one example, the battery contains 18,000 prismatic cells, and is designed as eight 1.25-MVA sub-units, each containing 13.8 kV-480 V switchgear plus inverter apparatus and associated battery bank. The proprietary controls decide in real time which of those sub-units to call upon to meet the ISO demand signals, such that battery cell life is maximized. Each of those 18,000 cells has its own voltage monitor and every other cell has a temperature monitor!

One interesting aspect of controls/automation is that they were too sensitive initially. According to Zents and Snyder, the system was so effective responding to frequency droop, the battery would sense small variations on the grid and respond with output. However, those responses unnecessarily counted as unit “starts.”

Starts are a precious commodity for a peaker in the LA Basin having to comply with South Coast Air Quality Management District (SCAQMD) emissions restrictions. Center is only allowed 356 annually. In the peak seasons, the unit could be called upon to start twice a day, so they can get tallied quickly.

The other big “pain point” during commissioning and early operation was the medium-voltage switchgear. One might think the associated relays, breakers, switches, and protection products would be off the shelf.

However, concedes GE’s application engineer for the project, Shanon Kolasienski, the design necessary for this first of a kind facility was not solidified far enough in advance. “Site details and relay protection schemes were not completely vetted ahead of time, so the equipment was shipped without secondary wiring, heaters, and communication devices,” he noted.

Zents and company expresses the medium-voltage switchgear headaches this way: “We took that hit for the next customers of hybrid EGTs.”

Beyond fast track

Given the schedule for this project, dictated in part by the anticipated emergency posed by SoCalGas’ Aliso Canyon, GE has been easily forgiven by the SCE project team. Other aspects of the project were deemed “lessons learned” but also originate in such an ambitious schedule. Zents and Kaushik agreed that air permitting did not start early enough. Local permitting issues, such as for safety and fire protection, were also challenges. After all, the local regulators had never experienced a project such as this.

Cybersecurity issues were also mentioned as an area for greater attention earlier in the project. Even though Center is low on the NERC CIPS asset classification scale, cybersecurity issues are still legion, especially with third-party vendors having access to the equipment for remote monitoring and maintenance.

But with two hybrid EGTS now under their belt, SCE is contemplating similar upgrades at the other three peaking facilities in its service territory. “From a revenue perspective, we could have selected any one of the five SCE peaker sites,” Kaushik said. In part, this is because spinning reserve is a zonal value, not a locational one. “Center posed the fewest complications in terms of necessary modifications, upgrades, footprint, permitting, etc,” he added, “so it was selected based on the cost side of the equation.”

Every molecule counts

It is common knowledge that every molecule of emissions avoided is important to the State of California. Thus, one of the great benefits of the hybrid EGT is that it is expected to cut the number of starts on the LM6000 by 50% and reduce overall run time by 60%, thereby avoiding the equivalent number of operating hours, associated emissions, fuel consumption, and noise.

This being the strictest air quality district in the country, that wasn’t enough. The LM6000 NOx and CO control systems were upgraded, with assistance from catalyst vendor Cormetech Inc, Durham, NC. By increasing the catalyst cross-sectional area (within the same outside dimensional footprint) and thereby exhaust gas-reagent contact area, and bolstering the strength of the aqueous ammonia concentration from 19% to 29%, the GT can now operate between 0.5 and 49 MW output and still meet a 2.5-ppm final NOx limit from the unit’s stack. The water injection algorithm was also modified, so that less water would be consumed.

This may seem like overkill to those in other parts of the country, but consider that the Center peaker site sits adjacent to a major transmission substation literally surrounded by commercial establishments, shopping centers, busy thoroughfares, and residential areas. The Target superstore just across the street is emblematic of the location.

Other commercial aspects

Although the power application (as opposed to an energy application) batteries were supplied by Samsung, GE holds the bundled performance and lifecycle guarantees on the hybrid EGT. SCE signed a 20-year LTSA with GE, with off-ramp opportunities after five years. The batteries are designed for a 20-year life.

The battery also adds a wrinkle to SCE’s resource adequacy (RA) requirements stipulated by the California PUC. RA means that SCE is obligated to maintain supply that is 115% above its summer peak demand. RA resources have to be able to deliver their maximum output for at least four hours. Dividing the BESS’ 4.3-MWh capability by the four-hour deployment period means that it contributes 1.075 MW of additional RA capacity.

To recap, the hybrid EGT adds these market services to the original non-spin reserve capability of the peaking GT:

  • Spinning reserve.

  • Regulation up, regulation down (battery can serve as load and supply).

  • RA resource.

By doing all this while significantly reducing fuel consumption and emissions unlocks additional value from SCE’s peaking GT fleet. Such capital investments for assets which will rarely operate more than 10% of the hours in a given year may seem surprising, but it reflects where electricity grid management is headed as more fuel-free but highly variable renewable resources are added.

One might wonder how CAISO “accounts” for the electricity needed to charge the battery. There are no meters onsite at Center to monitor parasitic power consumption. CAISO instead assumes a constant parasitic load that is factored into the real-time price it pays for the electricity. The BESS’ inverters (true for all inverters) constantly consume some electricity from the grid even while the hybrid EGT is offline, but that’s the nature of the component.

The bottom line, notes the SCE project team, is that the value of non-spin assets has declined, mostly because so many quick-start GTs, as peakers or part of combined cycles, can now be online within 10 minutes. Investing to make the SCE peaking units spinning reserves, which will show some diminishing returns across the fleet (the grid needs a limited amount of spinning reserves), justifies such investments in grid-scale storage. Flexibility while avoiding incremental emissions has extremely high value in California.

The next peakers to be upgraded may feature even larger batteries, which will only enhance all the aforementioned benefits, according to SCE.

As a final note, major maintenance overhauls on the LM6000 GT/G can be extended because it will operate fewer hours and the combustor will experience much less water injection, prolonging its life.